October 22, 2025

Loch-tober Creeps into Green Lake

Each October, we highlight the science and stories behind lake management across the region. After our time up north in Snohomish, we travel back to Seattle to the City’s iconic Green Lake. Let’s take a closer look at how science and collaboration are shaping the lake’s future.

Managing Invasive Aquatic Plants in Seattle’s Green Lake

Green Lake is a shallow lake in Seattle surrounded by Green Lake Park, which draws thousands of people daily from all over the city and beyond. The lake is an important recreational and aesthetic resource for visitors but with a long history of water quality and aquatic plant problems, requiring active management by Seattle Parks and Recreation (Parks) to meet high swimming, boating, and fishing demands.

Herrera scientists have been managing Green Lake for Parks since the early 1990s and regularly volunteer with the Friends of Green Lake by presenting at meetings, installing and maintaining the Taiga floating wetlands, and are currently constructing a frog pond to bring native amphibians back to the lake.

Aquatic Plant Problems

Green Lake was invaded by the invasive species Eurasian watermilfoil (milfoil) in about 1980 that covered over 85 percent of the lake and nearly wiped out all native plants by 1991. Parks conducted aquatic plant harvesting of crew lanes from 1991-2000 and used bottom barriers at the two beaches in some of those years because the use of aquatic herbicides was prohibited. Parks planted 777 grass carp in 2001 that ate almost all aquatic plants in the lake within a few years, with milfoil at only 4% cover in 2005.

A Herrera volunteer discovered a new invasive plant called Brazilian elodea (Egeria) during a milfoil beach cleanup in 2020. Herrera’s detailed plant survey in 2022 showed the lake contained only three invasive plant species with milfoil at 27% cover, Egeria at 20% cover, and fragrant waterlily at 2% cover. Recreation impacts were generally limited to less than 10% of the lake where milfoil grew to the lake surface but were projected to increase with expanded growth of these invasive species.

Aquatic Plant Management Plan

In 2025, Herrera lake scientists developed a plan to manage invasive aquatic plants in Green Lake to reduce impacts to recreation, fish, and wildlife. Herbicides were selected for cost-effective management because City policy recently changed to allow them, and they have been effective at Lake Washington beaches. The first phase of the plan was implemented in June 2025 with a ProcellaCOR treatment of milfoil at the beaches and crew lanes. This new, fast-acting, and low-toxicity herbicide was very effective. The lanes were cleared for the sprint canoe/kayak national championships in August, which was the first boating competition at the lake where no one complained about milfoil, according to Tamara Oki at the Green Lake Small Craft Center.

Next Steps

Next year, dense milfoil in shallow areas used by long-distance swimmers will be targeted. Fluridone, a slow-acting systemic herbicide, is planned for use when Egeria approaches the surface in the center of the lake, and imazapyr may be used to trim back the expanded waterlilies. Multiple methods for planting various native aquatic plant species were proposed to improve fish and wildlife habitat without interfering with recreation. In addition, Parks is planning to prepare a phosphorus inactivation plan for the lake to reduce the potential for increased toxic algae blooms, which are partially fueled by decayed aquatic plant matter.

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Contributing Author: Rob Zisette